The present invention relates to an etch for fabrication of integrated circuits
In particular, the present invention relates to selective etching of silicon oxides in integrated circuit fabrication processes.
It is very frequently necessary to etch silicon oxides preferentially with respect to silicon in integrated circuit fabrication. For example, when a contact is being cut through an interlevel dielectric such as phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG) to a level comprising silicon (e.g. to moat or to polysilicon), the etch must be selective in this respect.
Conventionally, to etch oxide preferentially with respect to silicon, a fluorine deficient plasma is used. Typically a fluorine to carbon atomic ratio of about 2 to 1 is used, together with a small fraction (e.g. 5% or less mole percent) of oxygen. If too much oxygen is used, it scavenges the carbon as carbon monoxide, and etching proceeds in a fluorine-rich regime, wherein both silicon and silicon dioxide are etched. Thus, chemistry such as CHF3 plus O2 could be used, with the CF3 flowed at a rate much larger than that of the O2, according to the prior art.
However, the present invention does not fit the prescription of the prior art at all, since (1) nitrogen trifluoride is used as the primary fluorine source (and preferably no carbon-bearing species is included at all); (2) the atomic composition of the etch gas mixture includes more oxygen than fluorine, and, in the presently preferred embodiment, oxygen is flowed at 10 times the rate of the nitrogen trifluoride. NF3 has been tried in many different plasma etch applications over the years, but no prior art suggests that a gas mixture which is mostly oxygen, with a little NF3 added, will etch oxide preferentially with respect to silicon.
Moreover, the prior art oxide-over-silicon preferential etches have inherent problems with residues. That is, there is always the problem of depositing polymeric residue in exposed areas, and the exact mass flow ratios and process chemistry and conditions must be painstakingly adjusted to get them within the narrow windows required for good selectivity without residue deposition. The oxide etch taught by the present invention does not leave residues, which is a tremendous advantage over the prior art.
This is particularly advantageous in contact etching, where deposition of residues at the bottom of the contact hole will not only degrade the contact resistance, but will introduce greater random variation in this parameter, i.e. will degrade process control.
This is also advantageous in the direct moat isolation processes, wherein the field oxide, instead of being grown in a pattern, is grown (or deposited) everywhere and then etched in a pattern. In this technology the oxide etch which clears the field oxide will contact the naked surface of the moat, so that any residue deposition may cause gate oxide defects.
Thus it is an object of the present invention to provide an etchant for etching silicon oxides preferentially with respect to silicon.
It is a further object of the present invention to provide an etchant, for etching silicon oxides preferentially with respect to silicon, which does not deposit residues.
Thus it is an object of the present invention to provide an etchant for etching silicon oxides preferentially with respect to metal silicides.
It is a further object of the present invention to provide an etchant, for etching silicon oxides preferentially with respect to silicides, which does not deposit residues.
The nitrogen trifluoride plus oxygen chemistry turns out to have surprising advantages. In particular, it provides astonishing selectivity of oxide over silicon, contrary to the teachings of the prior art. See, for example: Donnelly, V. M., Flamm, D. L., Dautremont-Smith, W. C., and Werder, D. J., J. Appl. Phys., 55, 242 (1984). For example, this chemistry gives 12 to 1 selectivity to etch phosphosilicate glass preferentially with respect to monocrystalline silicon. It gives 13 to 1 selectivity to etch PSG preferentially with respect to P-type polysilicon. It gives 2.2 to 1 selectivity of plasma oxide over silicon.
Thus, the nitrogen trifluoride plus oxygen chemistry taught by the present invention, which is inherently very resistant to residue deposition, has substantial advantages in this respect.
A notable characteristic of this etch chemistry is that it will attack organic materials, such as photoresist, rapidly. This is disadvantageous in many applications, and requires use of a hardmask process in most applications.
The present invention is most particularly advantageous, as the above selectivities show, when the oxide to be etched is PSG or BPSG.
According to the present invention there is provided:
A process for etching silicon oxides in integrated circuit structures, comprising:
providing a partially fabricated integrated circuit structure having at the surface thereof a dielectric comprising silicon oxides;
partially covering said dielectric with a patterned hardmask;
exposing said integrated circuit structure to a plasma in a gas comprising both oxygen and nitrogen trifluoride, the input gas to said plasma comprising more oxygen than nitrogen trifluoride.